1. Field of the Invention
The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a fiber amplifier based light source for use with such inspection systems.
2. Description of the Related Art
The demands of the semiconductor industry for wafer and photomask inspection systems exhibiting high throughput and improvements in resolution are ongoing. Successive generations of such inspection systems tend to achieve higher resolution by illuminating the wafer or reticle using light energy having shorter wavelengths.
Certain practical advantages may be achieved when illuminating the wafer or reticle with light of multiple wavelengths including UV-DUV wavelengths. Providing suitable lasers for high quality wafer and photomask inspection systems presents a particular challenge. Conventional lasers generating multiple wavelengths or relatively short wavelength light energy are typically large, expensive devices with relatively short lifetimes and low average power. Wafer and photomask inspection systems generally require a laser generally having a high average power, low peak power, and relatively short wavelength for sufficient throughput and an adequate defect signal-to-noise ratio (SNR).
Of the currently available lasers, excimer lasers are a source of short wavelength, high average power light. However, excimer lasers tend to operate at a slow repetition rate, such as in the range of 4 to 10 kHz. Repetition rates in this range tend to create extremely high peak power levels and can damage the wafer or reticle. Excimer lasers therefore cannot meet requirements for many precision wafer and reticle inspection systems.
Another option to provide adequate DUV power entails generating shorter wavelength light from a longer wavelength source. This wavelength alteration process is commonly called frequency conversion. Frequency conversion can be inefficient, and can require a high peak power laser to provide sufficient light to the frequency conversion crystal.
Using frequency conversion, a system generating adequate short wavelength power needs to generate sufficient long wavelength power. Generating sufficient long wavelength power can be challenging or impossible with a conventional laser. One method for increasing the amount of long wavelength power entails adding an amplifier to the original laser source. Laser amplifiers present certain issues including generally poor spatial profiles, nonlinear effects, and can experience an effect known as thermal lensing. Thermal lensing occurs when the refractive index of the gain medium varies with temperature, and the temperature of the gain medium always varies with the intensity of the light propagating through the amplifier. Controlling this variation in a high power laser with sufficient precision is difficult, if not impossible. When the temperature of the laser changes, the spatial profile of the beam tends to change. A non-Gaussian beam tends to degrade the frequency conversion process and can damage optical components.
In this environment, a laser based illuminator or illuminating arrangement having multiple wavelengths that include ultraviolet-deep ultraviolet (UV-DUV) wavelengths may provide benefits over previous types of illuminators. Such an illuminating arrangement operating at sufficient power levels to provide adequate SNRs that are relatively inexpensive, have relatively long lifetimes may be preferable to other illuminators depending on the application.
It would be beneficial to provide a system overcoming these drawbacks present in previously known systems and provide an optical inspection system illumination design having improved functionality over devices exhibiting those negative aspects described herein.